The invention describes a novel technology that can integrate so far separated power units of a drive system, e.g., of vehicles, ships, and aircraft. The traditional separate units that can be omitted or simplified in consequence include:
one or several DC/DC converters that adjust the battery voltage to one or several electric machines; one or several drive inverters that invert the DC voltage output of the one or more DC/DC converters or of at least one battery for one or several AC or multiphase electric machines; one or several chargers, which may include a converter, one or several step-down-converters to include low-voltage, such as a 12 V/24 V/48 V auxiliary power supply bus in cars; a battery management system that monitors, controls, and/or balances for instance, the state of charge, the temperature, the power of current load, and/or the ageing of sub-portions of the battery or of individual cells.
Electric trains, electric ships, and electric cars according to the state of the art, including hybrid electric, fuel-cell electric, diesel electric, and battery-electric versions, typically comprise at least one energy source and/or electrical storage element that provide the energy for at least one electric machine. Whereas earlier electric cars preferably used DC motors as electric machine, more recent vehicles implement multiphase AC motors, e.g., synchronous machines and induction machines, and can typically not be operated directly with the electricity provided by the available one or more energy source elements and/or one or more electrical storage elements. For instance, most electric storage elements use DC voltages, which is not compatible to the AC power the machine requires. Furthermore, the properties of the AC power for the machine, such as amplitude, frequency, and phase relationship with the rotor position determine the working point of the machine. The at least one separate power electronic converter of conventional drive trains converts the electrical power between the units, such as one or more electrical storage elements, one or more electric sources, or one or more electrical machines, and adjusts the properties of the electrical power. Power electronic converters convert, for instance, DC voltage into single-phase or into single-phase or multiphase AC voltage; DC voltage of one voltage into DC voltage with another voltage; or AC voltage to DC voltage. Often such power electronic converters can furthermore be current-controlled. If not further specified, the term converter will denote inverters, rectifiers, and converters in the following.
The prior art strictly separates electric sources as well as electric storage elements and power electronic converters as shown in the literature [M. Ehsani, Y. Gao, J. M. Miller (2007). Hybrid electric vehicles: architecture and motor drives. Proceedings of the IEEE, 95(4):719-728.]. This separation is usually also highlighted by separate casings for each separate unit. Prior art demonstrates that the separation of power sources, such as batteries, and power electronic units, such as inverters, in drive units is highly established and not subject to any doubt by persons skilled in the art as is supported by Rozman et al. in U.S. Pat. No. 8,536,729, Schleser et al. in US 2013/0317686, and Plant et al. in US 2014/0312619.
In many conventional electric drives that incorporate batteries, e.g., battery electric or hybrid cars, the involved batteries have usually a lower voltage than the installed electric machines would require for their peak power. In electric cars, for instance, battery-pack voltages of about 200 V to 400 V and peak machine voltages of about 600 V to 1000 V are frequently used, e.g., shown by Hatanaka in CA 2806817, Liu et al. in DE 10 2012 007 158, and Zhu et al. US 2003/0214826. The reasons for using a lower battery-pack voltage than peak machine voltage include safety and stability over a wide state-of-charge (SOC) range. As the battery pack in conventional applications is hard-wired and therefore presents the peak voltage at all times, also while the machine is not operated and/or the so system turned off, it is assumed to be a major safety risk and complicates the work of rescue teams in case of accidents. With respect to the SOC, manufacturers for example of electric vehicles expect the rated peak mechanical power and speed at the one or several machines even for low SOC, e.g. 20% to 40% SOC. Thus, a DC/DC converter can further compensate the reduced voltage of the battery to achieve the requirements of the machine.
The present invention allows eliminating such DC/DC stages. As a solution, the same number of battery cells as in conventional battery packs can be distributed among modules such that the sum of the module voltages for every phase is higher than the required phase voltage for maximum power in the machine, also when the SOC of the battery cells is low (e.g., 20% or 40%). If, for instance, a 3-phase machine is used with a peak voltage of 800 V together with the topology shown in FIGS. 3b and 5-7, the module voltages in every strand (also called module strand, arm, module arm) have to sum up to more than 800 V for the entire specified peak-power SOC range. Since traditional battery packs consist of hard-wired parallel and series cells, the number of total battery cells in the invention can stay the same.
The safety of the drive train can be improved, by switching the modules to a parallel, a bypass, or a passive mode/state at times when the machine is not operated. Thus, the highest voltage between any two electric contacts or metallic points in the entire system is the highest module voltage when the machine is not operated as described. Because the module voltage (preferably between 0 V and 200 V, particularly preferred 3 V to 60 V) is typically lower than the battery-pack voltage (typically in the range of 200 V to 400 V), this control rule together with the presented system is considerably safer than the conventional battery pack. In addition, magnetic components as required in the DC/DC stage can be omitted, reducing cost, space, and losses.
In conventional drive systems, an inverter, such as a number of push-pull half-bridge stages or more-level half-bridges, each formed by at least two semiconductor switches, provides the (usually alternating) voltage output for the at least one electric machine, each typically with at least one and preferably with three phases. In the invention, this function can be performed by a reconfiguration of the electrical interconnection of the modules and therefore the battery connectivity. In consequence, the invention can provide AC or multiphase voltage or current with practically free control over amplitude and frequency without the requirement of additional inverters. Furthermore, the source impedance of the invention compared is lower than that of a conventional battery if a parallel so mode/state, which connects the incorporated batteries of at least two modules in parallel, is provided by the modules and used in an appropriate control approach. With such parallel mode which can be used dynamically, the efficiency can be substantially increased compared to the state of the art.
In addition, in conventional drive systems, at least one separate charging unit (a) converts the power provided via an external charging connector (AC or DC) into DC voltage and current suitable for battery charging, (b) controls the power transfer, charging speed, charging approach/protocol (such as constant current, constant voltage or more complex charging approaches, as well as changes between phases of each), and (c) detects the end of charge. The charging unit usually incorporates a converter, which (a) increases cost, (b) requires space, and (c) increases weight. To save with respect to all three aspects, the charger is usually rated at a much lower power than the at least one electric machine, leading to long charging times.
Finally, while the one or several electric machines of conventional systems require relatively high voltages, many electrical loads, for example, lights, controllers, servos, driver assistance systems, and communication systems, commonly called auxiliaries, demand low-voltage power supply. Typically, 12 V, 24 V or 48 V are currently considered the standard for vehicles. Comparable rated voltage levels on ships, airplanes, and other vehicles deviate without loss of generality. In typical conventional systems, a DC/DC step-down converter allows exchange of energy between the drive battery and these auxiliaries. In vehicles, several units, such as the antilock braking system (ABS), the electronic stabilization program (ESP), and servo-assisted brakes can rapidly increase the current by more than 100 A, and are relevant for safety. Cost-effective DC/DC step-down converters could not follow such fast load fluctuations. Therefore, at least one conventional 12 V, 24 V, or 48 V car battery is often connected to the low-voltage side to filter and respond to respond to fast load fluctuations. However, this low-voltage battery (usually lead-acid technology) adds additional weight and cost.
The present invention splits so far typically hard-wired batteries with fixed connectivity, i.e., electrical connection of their subportions and/or cells in electrical series and/or parallel and/or bypass configurations, into at least two battery modules. A battery module according to the invention comprises at least one battery subportion and two electric switches, wherein said at least two electric switches allow a dynamic change of the connectivity of at least two battery subportions as described later. Such dynamic reconfiguration can provide at least one electric voltage of at least one electric terminal, e.g., an ac output, a dc output, or an arbitrary waveform output. Furthermore, such dynamic reconfiguration allows energy exchange between different battery modules to support, for instance, charge balancing.